As well known, high impact styrene polymers are obtained by bulk or bulk-suspension polymerization of solutions of polybutadiene or styrene-butadiene rubbers, or of saturated rubbers of the ethylene-propylene (EPM), ethylene-propylene-diene (EPDM) kind, in styrene alone or in admixtures of styrene with other polymerizable ethylenically unsaturated monomers. Generally, high impact styrene polymers, based on EPM or EPDM rubber, are obtained in the form of a masterbatch having a high rubber content, and are used either as such or in blends with stiff matrixes (such as for instance a styrene-acrylonitrile (SAN) copolymer or others) in order to obtain polymeric materials suitable for injection molding and thermoforming.
Such high impact polymers have excellent toughness properties at room temperature, particularly at high rubber concentrations, whereas the impact strength at low temperatures (for instance at -30.degree. C.) and the break elongation, in tensile stress tests, are not always satisfactory for all applications.
Such drawbacks may be avoided or limited to some extent by blending the high impact polymers with other polymers having the missing properties, in order to obtain a material having the desired combination of properties. Such approach, however, has been successful only in a few cases; in fact, the blending generally leads to the combination of the worst features of each component, thereby obtaining a material having such poor properties that it is of no commercial or practical value.
The main object of the present invention is to provide high impact styrene polymers containing an olefinic rubber, preferably an ethylene-propylene or an ethylene-propylene-diene rubber, having high values of toughness and flexibility and endowed with improved properties of impact strength at room temperature and at temperatures lower than room temperature, and of break elongation.